Aerial view of the Diablo Canyon Nuclear Power Plant, which sits on the edge of the Pacific Ocean at Avila Beach in San Luis Obispo County, Calif. It is an at-risk reactor like those in Japan.

The at-risk reactors are the Diablo Canyon Power Plant and San Onofre Nuclear Generating Station in California; the South Texas Project near the Gulf Coast; the Waterford Steam Electric Station in Louisiana; and the Brunswick Steam Electric Plant in North Carolina.

They appear in an analysis by the mapping and geographic data firm ESRI Inc., based in Redlands, Calif. The online map, the first of its kind to let the public search potential danger zones by address, includes U.S. Geologic Survey (USGS) seismic information and earthquake history for every nuclear plant in the USA.

After the Fukushima disaster, President Obama ordered the Nuclear Regulatory Commission to evaluate the earthquake risk of every nuclear plant in the nation, said Victor Dricks, an NRC spokesman. Dricks said NRC regulations require companies that build nuclear plants to take into account local seismic history and fortify the plants against the largest quake that is likely to occur.

Dricks said the U.S. has taken proper precautions to ensure the safety of its plants. San Onofre, for instance, is built to withstand a magnitude-7.0 earthquake within 5 miles of the site, he said. In addition, the plant is 30 feet above sea level and has a reinforced concrete sea wall that is 30 feet tall and could withstand a 27-foot tsunami.

Japan's Fukushima Dai-ichi plant suffered major damage from a magnitude-9.0 earthquake and 46-foot tsunami that hit March 11. The disaster triggered nuclear radiation leaks and an extensive evacuation in the region around the plant, which was built to withstand a 19-foot tsunami.

The ESRI map aims to help Americans determine their risk. It allows users to plug in their location and find the five nearest nuclear plants.

Users can also determine whether they live within 10-mile or 50-mile U.S. evacuation zones of any nuclear plants and whether the region around the plant has been jolted by any major earthquakes, measuring magnitude-7.2 or higher, in the past 30 years.

"All of the earthquakes on this map are significant," said ESRI analyst Bronwyn Agrios, noting that the analysis was eye-opening for those on ESRI's staff. "We found that we're just on the cusp of the evacuation zone of the San Onofre plant, just down the coast on the ocean side. Right around our area there have been three earthquakes. We're in a highly dense area for faults. We can feel that. We can feel tremors every week."

William Leith, acting associate director for natural hazards at the USGS, said it's impossible to predict the precise timing, location and magnitude of an earthquake, in part because quakes have been measured in the USA only for a century.

Although most nuclear plants are in the central and eastern USA, where earthquakes are rare, the USGS ranks 39 states as having a high or moderate earthquake risk, Leith said. New studies have shown that at least 20 magnitude-9.0 earthquakes have struck off the coast of Northern California, Oregon and Washington in the past 20,000 years, most recently in 1700, he said.

"We don't want to alarm anybody," he said, "but it can happen here."

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